As the method for removing cesium ions from cooling water of a nuclear power plant or polluted water in soil, which are contaminated with radioactive cesium such as 137Cs and 134Cs, there is a method of adsorbing cesium ions onto a support that is, for example, zeolites such as mordenite, heteropolyacid salts such as ammonium phosphomolybdate, acidic salts of a polyvalent metal such as titanium phosphate, or an inorganic ion exchanger such as insoluble ferrocyanides (Patent Documents 1 and 2 and Non-Patent Document 1).
However, these supports are low in the selectivity for cesium ion and when a seawater metal ion, particularly sodium ion, is present together in the aqueous solution, the adsorption performance for cesium ion is extremely decreased, as a result, a large amount of an adsorption support is required and in turn, a large amount of radioactive waste must be treated. Also, since a support insoluble in water is used, the cesium ion adsorption reaction is inhomogeneous and a lot of time is taken to reach an adsorption equilibrium.
Also, there is a method of directly charging insoluble ferrocyanides into water contaminated with radioactive cesium to adsorb cesium ions, forming a precipitate by using a polymer flocculant, centrifugally separating the precipitate, and through filtration under reduced pressure and drying step, removing the radioactive cesium in a short time (Non-Patent Document 2).
In the method of directly charging insoluble ferrocyanides, the treatment may be performed in a short time as compared with the conventional method using a support, but a lot of time is still taken and also, automation of the process involving filtration under reduced pressure is difficult, as a result, the number of human-performed steps of treating the precipitate containing radioactive cesium concentrated to a high concentration increases, giving rise to a serious problem that the opportunity to be exposed to radiation while doing this treating work is increased.
Furthermore, in recent years, a wastewater treating system using a magnetic particle has been developed, and magnetic separation is utilized for the removal of heavy metals in an aqueous solution. Accordingly, when this method is utilized for the separation of cesium ions in an aqueous solution, cesium ions can be removed by magnetic separation without steps of centrifugation and filtration under reduced pressure and therefore, this method is supposed to be suitable for rapid removal of cesium ions with lessened exposure of human to radiation.
For example, there is a known method of previously preparing a solid adsorbent composed of a magnetic particle having bonded thereto iron ferrocyanide, charging the adsorbent into water contaminated with radioactive cesium to adsorb cesium ions, and separating the solid adsorbent to which cesium ions are adsorbed, by using a column placed in a magnetic field (Non-Patent Document 3). The method of separating the solid adsorbent and water by magnetic separation can realize separation in a short time and treatment with a relatively compact facility. However, this method requires pre-production of a magnetic particle having bonded thereto iron ferrocyanide and since also in this method, a magnetic particle insoluble in water is used as the solid adsorbent, a lot of time is taken to reach an adsorption equilibrium, making its practical use difficult.